Sound transducers convert electrical signals into sound waves. A loudspeaker has one or more output sound transducers supported in a housing. The housing has a front opening for each output transducer. The volume of space behind the output sound transducer is the speaker cavity. Each sound transducer has a diaphragm that vibrates in response to the amplitude and frequency of applied electrical signals. With all passive designs, the shape and the size of the cavity influence the output of transducer. Under normal atmospheric conditions at frequencies above a few hundred hertz even a small cavity can be used to trap and prevent out-of-phase sound waves produced by a speaker, or, in general, any sound transducer, from interfering with the desired waves. But at frequencies below a few hundred hertz, the enclosure volume and resonance effects become significant. At low frequencies the size of the cavity creates pressure effects that alter the transducer""s output compared to the ideal enclosure. The pressure effects create sound output amplitude decreases with lower frequency (roll off), distortion with decreasing frequency, and unwanted resonances. Present speaker designs generally rely upon passive acoustic methods to compensate for enclosure effects on the transducer output. Some examples of passive compensation include acoustic suspension, bass reflex, or the use of materials such as fiberglass to increase the effective cavity size. Another design uses the phase effects of several radiating speakers sharing the same enclosure to alter normal passive effects produced by the enclosure. Since accurate and loud base reproduction is hard or impossible to achieve with simple passive designs, the designers use the resonance effects to create a xe2x80x9cbooming soundxe2x80x9d or false base that is not an accurate reproduction but has the sound of loud low sounds.
In a passive design, the inherent qualities of the structure of the design are used to counteract roll off and resonance without expending energy to artificially control cavity characteristics. Examples of such passive designs are:
1. Cavity size can be increased so that roll off is experienced at lower frequencies.
2. The cavity is filled with material, such as fiberglass, to increase the effective acoustical size of the cavity.
3. Special dampening materials can be used to reduce structural vibrations.
Despite efforts of others to passively control the cavity pressure, there remains an unfulfilled need for small speakers that can accurately reproduce sound, especially low frequency sound. There is a need for a speaker that will minimize low frequency distortion. There is also a need for a speaker that will reinforce certain frequencies to provide resonance at one or more desired frequencies. There is a further need for a speaker whose output is adjustable to both null certain frequencies and to reinforce others. There is a need for a speaker that is adjustable so the ranges of the nulled and reinforced frequencies are not fixed, as in passive designs, but are controllable and variable as selected by the user. There is a need to control these properties independent of constraints on the cavity""s shape and volume. These and other needs are met by the invention described and claimed below.
Some prior art attempts have sensed the pressure in the cavity behind the output transducer and have used negative feedback to null the pressure in the output cavity. See, for example U.S. Pat. Nos. 5,461,676 and 5,327,304. However, it is difficult to accurately provide a null closed loop feedback system that has acceptable distortion. often the distortion of such systems are unacceptable to listeners. Moreover, such closed loop null systems can require complex control electronics. Even when such systems are used, they are unstable at certain frequencies that cause unwanted oscillations. It has been noted that even when pressure is nulled, the cavity will continue to vibrate. This indicates the need for positive pressure to reduce the spurious vibrations. Such systems cannot be adjusted to provide positive pressures or artificial resonances at arbitrary frequencies and rely only on nulling the pressure in the output cavity.
U.S. Pat. Nos. 5,461,676 and 5,5,327,304 apply to the restricted case of closed loop cases only. The above patents make no reference or implications to any of the other more general cases of active resonances and minimums, open loop designs, static pressures other than equal to outside, other gases, or the infinite cavity. The latter is not in a nulled condition but consist of traveling waves of similar amplitude but 180 degrees out of phase with the output. They also do not infer simulating passive effects such as resonances and minimums, or other active effects not possible with passive designs in the general sense of total arbitrary control of the cavity pressures and their effects on the speaker output. Closed loops, while helpful, are nevertheless restrictive in the amount and types of controls that can be applied to the speaker cavity. The cited patents are limited to nulling cases. However, full control of the cavity would allow the user to selectively increase or decrease the pressure in the speaker cavity as a function of variables other than cavity pressure, e.g. the frequency of the driver signal. Applicant""s open loop design provides such flexibility.
The effects of FIGS. 3.2, 3.3 and the example design of FIG. 5.1 are not suggested in the two previous patents. The example design of FIG. 5.1 is totally open loop in operation. The sensors previously shown in the parent application were used only in characterizing the design. None of the data displayed in FIGS. 6 and 7 for the normal operating mode of the speakers are made with a closed loop or nulled cavity. The example design relies on the principles of artificial resonances and minimums only. These effects are in no way mentioned of implied in the previous patents. The circuit of FIG. 7.4 which produces the necessary phase shifts, is part of an open loop system with no sensor input. Setup of the adjustments shown is done once initially using only the sound level at the listener in the specific environment in order to produce the flat or other desired final response. The phase settings are different for different amplifiers and acoustic effects of different listening environments. The data of FIG. 6 was made at single frequencies with the open loop circuit C1, the same circuit used to produce the flat responses of FIGS. 7.1 and 7.3.
The invention reduces or eliminates the xe2x80x9cbooming soundxe2x80x9d due to resonances. That result is in not implied, mentioned, or obvious in the art of record. In contrast, it was determined experimentally. It is not obvious or true that all other active designs other than the example will eliminate the booming sound in all cases and it is definitely not true if the resonance frequency is not specifically driven. The open loop design was initially chosen because a closed loop design must use complex digital electronics to prevent oscillations and instabilities if a good null or other active effects was to be obtained, due to the complex phase and amplitude changes occurring in the active cavity. The closed loop oscillations that occur using an analog circuit could not be filtered out since they are contained within the desired band. In contrast, the open loop designs can be very simple and practical to produce cheaply, as the design shown, C1 which uses a single integrated circuit. In practice C1 is just connected in series with one of the existing subwoofer amplifiers, is adjusted once on setup, and no other wires such as those for sensors being required. The normal dual subwoofers are then replaced with the single active design. The original dual channel subwoofer amplified is used to drive the single active subwoofer.
The U.S. Pat. No. 5,327,504 of Jul. 5, 1994 and U.S. Pat. No. 5,461,676 of Oct. 24, 1995 apply only to speaker with a closed loop null with cavity pressure equal to the outside, i.e. closed loop nulling. The filter method will not work, at least in the example cases of FIGS. 5.1 and 12, because resonances occur within the band of interest 20-100 Hz, so filtering would delete part of the frequency band. In addition analog closed loop instabilities cover a wide range of frequencies so a filter would have to delete at least a third of the band. The mechanical resonances of the active cavity are more complex than its electrical counterpart because both the phases and amplitudes needed for resonance vary depending on cavity pressure conditions. So even if the phase is artificially shifted at one resonance to cancel it, the cavity always seems to find a new resonance if using sufficient gain to close the loop. Even if a stable region could be found in the control loop, complex music with its widely varying amplitude and frequencies could drive an apparently stable circuit into temporary oscillations when conditions sometime force the control loop into a region of instability.
In normal practice the nulled cavity would not be used much, but more likely some combination of maximums and minimums, with possibly a few specific frequencies only using the nulled cavity. The closed loop null cavity is not as practical as the non nulled and open lop case because:
1. It does not produce the usual desired response, which is flat from 20 Hz to some higher value in the 100 to several hundred Hz range.
2. It is not practical to implement using simple analog electronics over the normal frequency ranges desired.
3. Complex digital electronics are needed to produce a total cavity null over all frequencies normally of interest, making the circuits expensive unless produced in very large numbers. Even if digital was implemented it would most likely be used to produce other effects in addition to the simple null.
4. The simple cavity null does not produce the lowest distortion at all frequencies depending on the speakers used. A positive force on the speaker cone, especially at the lower frequencies, about 25 Hz and less, helps prevent oscillations of the speaker cone material. At very low frequencies oscillation modes can be excited on the cone material itself due to the lack of pressure due to the low speed of the cone. These will emit directly into the outside appearing as distortion. This can even happen at higher frequencies which is one of the reasons the cavity may often be run with carefully controlled pressures that do not interfere with the desired output.
5. Some speakers become mechanically offset at high continuous powers, which can be corrected for with a static pressure different than the outside pressure.
The invention provides an actively controlled sound transducer cavity and a method for actively controlling the sound energy in the cavity and, in particular, controlling changes in pressure in the cavity. A speaker having an open loop, nonnull closed loop or a combination of open and closed loop controls for controlling the pressure in the cavity. In particular, the controls use the output driver signal as modified by one or more controls for changing the phase, amplitude or frequency, including one or more harmonics of the driver signal. The invention allows the user to select ranges of frequencies for nulling and for reinforcing, and controlling the distortion effects of passive cavities. The speaker comprises an enclosure with one or more apertures. In each aperture is as output sound transducer, typically a diaphragm, which radiates output sound into the ambient space in front of the loudspeaker. When the diaphragm radiates output sound, it also produces spurious sound waves in the cavity. The spurious sound waves alter the pressure in the cavity.
One or more control means are mounted in the cavity to actively control the pressure in the speaker cavity. The actively controlled cavity is independent of the size and shape of the cavity. In one embodiment the control means are cavity pressure control devices (CPCDs). The CPCDs are mounted in the cavity and sound waves or pressure in the cavity that can null or reinforce a spurious sound wave or pressure changes produced by the output diaphragm, or produce other desired pressure effects. So, the cavity pressure control devices are also sound transducers, or other devices for altering pressure in the cavity.
The control means may be driven by an open or closed loop device. In the open loop configuration power is applied to the CPCD in a predetermined manner depending on the input signal. In the closed loop configuration the power applied to the CPCD depends on the pressure in the cavity. A hybrid configuration uses a combination of open and closed loop to drive the CPCD.
For a closed loop configuration the control means may includes one or more sensors. The sensors detect the effects of instantaneous changes in pressure in the cavity produced by spurious sound waves. The sensors are mounted in the cavity and they sense characteristics of the spurious sound waves including phase, frequency, and amplitude. A typical sensor is a microphone. The sensors are coupled to a control device (CD). The CD generates a control signal that operates the CPCD.
For an open loop system the CPCD is driven in a predetermined fashion with a phase, frequency, and amplitude to produce the desired transducer output effects, independent of the actual pressure in the cavity. The open loop configuration does not depend on a cavity sensor for operation. In the open loop mode the CPCD will usually be driven with a phase, amplitude and frequency that is related to the transducer input signal, although it may also be independent of the input signal depending on the desired effect.
For a hybrid closed/ open loop system the power to the CPCD is both predetermined and to some extent related to the cavity pressure. This configuration is more practical for very precise cavity control since a closed loop system can fine tune the pressure after most of the pressure alteration is completed by the open loop system.
For both open and closed loop systems the phase, amplitude and frequency of spurious cavity sound waves are important characteristics. For closed loop sensing the phase, amplitude, and the frequency of the cavity sound waves, the CPCD can be driven with the correct phase, amplitude, and the frequency so as to produce the desired cavity pressure. The CD may or may not use the same driver signal applied to the output transducer to drive the CPCD. In general by phase shifting the driver signal and altering its amplitude, the unwanted frequencies of the spurious sound waves may be mostly nulled, increased, or decreased. The CPCD driver signal has to be phase shifted and amplified before it is applied to the CPCD. To obtain a good null or low distortion transducer output harmonics usually have to be added to the CPCD driver signal, also with the correct phase and amplitude. A closed loop system will automatically generate the correct signals, where as in an open loop system these are predetermined.
This invention actively alters the cavity pressure to artificially produce arbitrary cavity waves or pressure effects. Thus any passive design may be artificially simulated to the extent allowed by the designs and components used. In addition, other effects may be produced that are not generally practical using passive techniques. For instance, resonances at specific frequencies or ranges of frequencies, minimums, the simulation of the ideal enclosure or nulled cavity and most importantly the flat/ low distortion response.
In the active control design of the invention, work is performed in a controlled manner to accomplish a desired function. Examples of this, within the scope of the invention, are the following: (1) one or more cavity transducers are used to null the pressure in a speaker enclosure, producing a lower roll off frequency, less distortion, and absences of resonances; (2) one or more cavity transducers are used in order to control the pressure in a small speaker enclosure producing the effect of a much larger or ideal enclosure size; (3) one or more cavity transducers are used in order to produce a response that is flat with low distortion from 10 to 100 Hz in a small speaker enclosure. So, the invention provides a speaker of relatively small dimensions or arbitrary shape that faithfully reproduces sound at the lower ranges of human perception.